Normal fluid requirements

Normal people balance their daily intake and output. Adults exchange approximately 5% of their body water each day. Normal maintenance fluid requirements are calculated as follows:

So a 70 kg man requires 2640 ml per day - which is where the commonly used 3 litres a day comes from (Table 5.1).

Maintenance fluid covers insensible losses plus the required intake to maintain normal fluid balance (like replacing urine output). Insensible losses include things like sweating and breathing and are around 1200 ml per day. The maximum concentrating power of the kidney is 1200 mosmol kg-1 so a minimum of 500 ml urine is needed to excrete the daily solute load of 600 mosmol. Oliguria is defined as a urine output of

Table 5.1 Maintenance fluid according to weight

Weight (kg) ml per hour ml per 24 hours

50 90 2169

60 100 2400

70 110 2640

80 120 2880

90 130 3120

100 140 3360

< 0 5 ml kg-1 per hour for two consecutive hours or < 400 ml in 24 hours. The elderly have a reduced ability to concentrate urine because of age-related nephron loss.

Electrolytes are needed as well as water: 50-150 mmol sodium and 20-40 mmol potassium is the average daily requirement. An average water and sodium requirement can be found in 1 litre 09% saline and 2 litres 5% dextrose per 24 hours; however, note that this is maintenance fluid in a normal person.

Accurate replacement of fluid depends on understanding the distribution of water, sodium, and colloid in the body (Figure 5.1). Humans are 60% water. Females have less because of more fat (which contains less water). Total body water is up to 80% in neonates falling to 45% in the elderly. Two-thirds of total body water is in cells (ICF) and the remaining one-third is in the extracellular compartment (ECF). Sodium is the major extracellular cation and, with chloride, provides the effective osmolality (or tonicity) of this compartment. Potassium and phosphate are the major intracellular ions (Table 5.2). The ICF and ECF are separated by a semi-permeable membrane, which allows free movement of water. Movement of sodium and glucose results in movement of water between the ICF and ECF.

Serum osmolality is regulated by osmoreceptors in the hypothalamus. Osmolality is a measure of the total number of particles in a given weight of plasma. Any minor increase (> 2%) is sensed by these receptors and this causes the release of antidiuretic hormone (ADH), which acts on receptors in the collecting duct, opening aquaporin channels and causing water reabsorption with the excretion of a concentrated urine. A reduction in serum osmolality normally inhibits the release of ADH leading to more dilute urine. A normal 70 kg man

Osmotic equilibrium at the cell membrane (water passes freely). Electrolytes pass via an energy-dependent process

Cell membrane Endothelium r

3 litres

Fluid movement here via Starling forces: hydrostatic (pushing)

v osmotic (pulling) pressure gradients

25 litres

14 litres

3 litres

PLASMA

<-Intracellular

Figure 5.1 Water distribution in the body (of a 70 kg man). Sodium changes in the extracellular compartments cause water shifts to and from cells, contracting or expanding the ICF. The third space is a non-exchangeable compartment in the ISF

Table 5.2 Electrolyte contents of different body compartments

(in mmol litre-1)

Compartment

Sodium

Potassium

Chloride

Bicarbonate

Plasma

142

4

1G3

27

Interstitial

144

4

114

3G

Intracellular

1G

15G

G

1G

filters 140 litres of water at the glomerulus per day and the majority (70%) is reabsorbed in the proximal convoluted tubule. ADH is controlled to a lesser extent by baroreceptors located in the heart and great vessels. In the presence of volume contraction, ADH is released.

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